1. Field of the Invention
This invention relates to a secondary zinc electrode particularly, but not exclusively, for use in a nickel/zinc or silver/zinc electrochemical cell.
To date, zinc electrodes used in rechargeable electrochemical cells have suffered from a number of shortcomings, one of the most serious of which is a tendency for the zinc to migrate from the periphery to the centre of the electrode. This phenonenon, which is generally referred to as "shape change," leads to the edges of the electrode becoming inactive and also reduces the activity at the centre of the electrode by causing the density of the zinc at the centre to increase, A further problem associated with secondary zinc electrodes is that, during charging, metallic zinc produced by ion reduction is deposited on the electrode in the form of dendrites which grow in a direction substantially perpendicular to the electrode. This growth may occur through the separators provided between the positive and negative electrodes in an electrochemical cell and thereby cause internal short circuits.
2. Description of the Prior Art
Among the solutions which have been proposed to the problem of shape change is the incorporation of an organic binder, particularly polytetrafluoroethylene, in the electrode. However, although the solutions so far proposed have resulted in some reduction in the amount of shape change, they have not proved entirely statisfactory. One particular problem in this context is that, when polytetrafluoroethylene alone is incorporated in the electrode, the polytetrafluoroethylene tends to coagulate as a film on the outside of the electrode during cycling. This not only reduces the useful effect of the binder in avoiding shape change but also increases the resistance of the associated electrochemical cell. Another proposal for reducing the problem of shape change is to produce highly porous (i.e. having at least 60% open pores) zinc electrodes. However, this not only results in an increase in the volume of the electrode and hence a decrease in its volumetric energy density, but also reduces the strength of the electrode.
French Pat. No. 2264401 describes an alternative form of secondary zinc electrode which is produced by applying to a collecting grid a non-hardened mixture containing particulate zinc oxide, a binder such as polytetrafluoroethylene, polyvinyl alcohol, polypropylene, polyethylene or carboxymethyl cellulose and other substances, such as rayon fibres or metal powders. However, this electrode still suffers from the problems of dendritic growth and, in particular, shape change or deformation, probably as a result of movement of the electrolyte parallel to the surface of the electrode during the charging and discharging cycles. As a result, the service life of this electrode is very short.
Yet another problem experienced with conventional secondary zinc electrodes is that, with repeated discharging and charging cycles, the crystal size of the zinc increases, which contributes to a rapid decrease in the capacity of the cell.